Polar Electrojet Research Articles

Near-field Response in Lossy Media with Exponential Conductivity Inhomogeneity

This paper examines the near-field response to source currents in lossy media with exponential conductivity inhomogeneity. The motivation for this work is to understand the modification of the polar ionosphere D region (50–90 km altitude) by powerful high frequency transmitters. The transmitted waves heat the D region plasma, causing a localized conductivity perturbation. In the presence of the DC electric field of the polar electrojet, the conductivity perturbation produces a current perturbation referred to as "antenna current" that can drive extremely/very low frequency radiation. Here we seek to understand the production of antenna current in a strongly inhomogeneous plasma. In the lower D region, the static approximation is valid, and we solve using a scalar potential description. In the upper D region, we use the magnetoquasistatic approximation and solve using a vector potential approach.

Quantitative analysis of the relationship between polar ionospheric currents and auroral electrojet indices

The ionospheric currents in the polar region are caused mainly by field-aligned currents. The quiet polar current system consists of a pair of current vertices at dawn and dusk. When substorm occurs, however, intense magnetic disturbances are generated by enhanced polar currents, especially the westward electrojet of a millions Amperes in the auroral oval. The intensity of the auroral electrojet is commonly described by the auroral electrojet indices AL , AU , AE and AO . In this paper the relationship between the polar current system and the auroral electrojet indices is quantitatively studied by means of polar current functions obtained from the data recorded at 71 magnetic stations in the northern polar region during the International Mag-netospheric Study (IMS). Fairly well linear correlation of AL (or AU ) with AE index has been found, indicating that AE index multiplied by a proper factor can approximately substitute AL and AU indices. The total polar current, J T , and the strengths of the dawn and dusk current vortices, J T 1 and J T 2 , can be characterized by AE index, an increment of 1nT in AE index corresponding to 1000A in J T . A fairly well direct proportion is found between AE and the maximal westward current density, an increment of 1 nT in AE corresponding to 1 A/km of the maximal westward current density at magnetic midnight. The latitudinal profiles of the current density are similar for different local time in night sector. The maximal density of the westward electrojet usually occurs at geo-magnetic latitude 65°—70°around midnight, as for the eastward electrojet, it is around 80°. The analysis shows that for 5-mimute mean values, the saturation values of AE and AL are 700 nT and –500 nT, respectively. Accordingly, a caution should be taken when the indices greater than the saturation values would be used for studying magnetospheric or ionospheric processes.

Concerning long-term geomagnetic variations and space climatology

Abstract. During geomagnetic polarity transitions the surface magnetic field of the Earth decays to about 25% and less of its present value. This implies a shrinking of the terrestrial magnetosphere and posses the question of whether magnetospheric magnetic field variations scale in the same manner. Furthermore, the geomagnetic main field also controls the magnetospheric magnetic field and space weather conditions. Long-term geomagnetic variations are thus intimately related to space climate. We critically assess existing scaling relations and derive new ones for various magnetospheric parameters. For example, we find that ring current perturbations do not increase with decreasing dipole moment. And we derive a scaling relation for the polar electrojet contribution, indicating a weak increase with increasing internal field. From this we infer that the ratio between external and internal field contributions may be weakly enhanced during polarity transitions. Our scaling relations also provide more insight on the importance of the internal geomagnetic field contribution for space climate.

Model of anomalous electron heating in the E region: 1. Basic theory

Numerous radar observations demonstrate anomalously strong electron heating in polar electrojets during magnetic storms. The effect often correlates with the onset of the Farley‐Buneman instability caused by strong convection electric field. Anomalous heating of electrons is usually attributed to turbulent electric fields developing in the E region due to that instability. We propose a theoretical model of the effect based on heuristic assumptions on nonlinearly saturated turbulence and equations of the electron and ion energy budget. The model has been quantitatively tested in the companion paper by Milikh and Dimant [2003]. Good agreement with observations supports the implied physical mechanism.

Kinetic model of electron heating by turbulent electric field in the E region

Numerous radar observations demonstrate anomalously strong electron heating in polar electrojets during magnetic storms. The effect correlates with strong convection electric field. Anomalous heating of electrons is caused by turbulent electric fields developing in the E region due to the Farley‐Buneman instability. A quantitative model of the effect is proposed. Numerical computations are based on a kinetic code. A good agreement of the theoretical results with the existing observations supports the physical ideas underlying the model.

Generation of extra and very low-frequency (ELF/VLF) radiation by ionospheric electrojet modulation using high-frequency (HF) heating waves

Extra and very low-frequency (ELF/VLF) wave generation by modulated polar electrojet currents is studied numerically. Through Ohmic heating by the amplitude-modulated high-frequency heating wave, the conductivity and thus the current of the electrojet are modulated accordingly to set up the ionospheric antenna current. Stimulated thermal instability, which can further enhance the electrojet current modulation, is studied. It is first analysed analytically to determine the threshold heating power for its excitation. The nonlinear evolutions of the generated ELF/VLF waves enhanced by the instability are then studied numerically. Their spectra are also evaluated. The field intensity of the emission at the fundamental modulation frequency is found to increase with the modulation frequency in agreement with the Tromso observations. The efficiency enhancement by the stimulated thermal instability is hampered by inelastic collisions of electrons with neutral particles (mainly due to vibration excitation of N2), which cause this instability to saturate at low levels. However, the electron inelastic collision loss rate drops rapidly to a low value in the energy regime from 3.5 to 6 eV. As the heating power exceeds a threshold level, significant electron heating enhanced by the instability is shown, which indeed causes a steep drop in the electron inelastic collision loss rate. Consequently, this instability saturates at a much higher level, resulting to a near step increase (of about 10–13 dB depending on the modulation wave form) in the spectral intensity of ELF radiation. The dependence of the threshold power of the HF heating wave on the modulation frequency is determined.

Multisheet modelling of the electrical conductivity structure in the Fennoscandian Shield

Electromagnetic multisheet modelling is a powerful tool for large model areas, if they can be approximated by a multilayered heterogeneous conductivity structure of small vertical dimension in comparison with the penetration depth of electromagnetic fields. In this paper, thin sheet technique is applied to the whole Fennoscandian (Baltic) Shield, whose upper mantle conductivity structure is the objective of the long period electromagnetic array experiment BEAR (Baltic Electromagnetic Array Research). Three thin sheets, each of about 120,000 model cells with the base length of 10 km, describe a-priori crustal inhomogeneities in terms of conductances. The three sheets represent i) upper crust from surface to the depth of 10 km including continental and ocean bottom sediments and seawater, ii) middle crust ranging from 10 km to 30 km and iii) lower crust from 30 km to 60 km. Thus, modelling is taking into account distortions caused by crustal conductivity anomalies. Additionally, distortions due to inhomogeneous external current systems are investigated by introducing an equivalent current system of a polar electrojet model at ionosphere height. Modelling results are illustrated by induced current distribution at different depth levels and by various electromagnetic transfer functions. The latter demonstrate the resolution of crustal conductivity anomalies and their screening effect even at long periods. The predicted behavior of transfer functions in the very complex conductivity structure is compared with the experimental BEAR data, showing qualitatively a first order agreement for most of the sites. Modeled phases for periods of a few thousands of seconds are considerably biased in comparison with experimental data if the background 1-D model has monotonously decreasing resistivity. However, the bias from phases is removed if a conducting asthenosphere having a resistivity of 20 Ωm is emplaced between the depths of 200 km and 300 km. Thus, multisheet modelling indicates a well conducting upper mantle under the Fennoscandian Shield. All modelling has been performed using a multisheet code by Avdeev, Kuvshinov and Pankratov.

Major enhancement of extra-low-frequency radiation by increasing the high-frequency heating wave power in electrojet modulation

Extra-low-frequency (ELF) wave generation by modulated polar electrojet currents is studied. The amplitude-modulated high-frequency (HF) heating wave excites a stimulated thermal instability to enhance the electrojet current modulation by the passive Ohmic heating process. Inelastic collisions of electrons with neutral particles (mainly due to vibrational excitation of N2) damp nonlinearly this instability, which is normally saturated at low levels. However, the electron’s inelastic collision loss rate drops rapidly to a low value in the energy regime from 3.5 to 6 eV. As the power of the modulated HF heating wave exceeds a threshold level, it is shown that significant electron heating enhanced by the stimulated thermal instability can indeed cause a steep drop in the electron inelastic collision loss rate. Consequently, this instability saturates at a much higher level, resulting to a near step increase (of about 10–13 dB, depending on the modulation wave form) in the spectral intensity of ELF radiation. The dependence of the threshold power of the HF heating wave on the modulation frequency is determined.

Stimulated thermal instability for ELF and VLF wave generation in the polar electrojet

Generation of ELF and VLF waves in the HF heating wave modulated polar electrojet is studied. Through the Ohmic heating by the amplitude‐modulated HF heating wave, the conductivity and thus the current of the electrojet is modulated to set up the ionospheric antenna current. However, it is shown that a stimulated thermal instability is also excited by the amplitude‐modulated HF heating wave. This instability introduces an electron temperature modulation more effectively than that by the passive Ohmic heating process and is expected to improve considerably the intrinsic efficiency of ELF and VLF wave generation by the amplitude‐modulated HF heating wave. Moreover, the generation efficiency and signal quality also depend on the HF wave modulation scheme. Thus, four amplitude‐modulation schemes are examined and compared.

A new mechanism of whistler wave generation by amplitude modulated HF waves in the polar electrojet

It is shown that a spatially distributed mode current of whistler waves, oscillating at the modulation frequency of the ground‐transmitted powerful HF wave, can be induced in the E‐region of the high‐latitude ionosphere through the coupling between HF wave‐modulated electrojet current and induced density irregularities. As shown in Kuo et. al. [1999], these density irregularities generated by the HF wave via a thermal instability vary periodically along the geomagnetic field. This current produces whistler waves directly rather than through an antenna radiation process. This mechanism generates whistler waves in the VLF range (3–30 KHz) with reduced harmonic components. The frequencies of produced whistler waves depend on the polarization, frequency, power, and modulation scheme of the HF wave. It is predicted that whistler waves at frequencies around 25 KHz can be most favorably excited, awaiting the corroboration of future ionospheric heating experiments.

Simulations of ELF radiation generated by heating the high‐latitude D region

Three‐dimensional simulations show the ELF (extremely low frequency) radiation generated by heating the high‐latitude D region and modulating the polar electrojet The simulations use a time‐varying current perturbation in the D region. The ELF radiation is calculated in the Earth‐ionospheric waveguide to a radial distance of 2400 km. The angular and radial dependence and the polarization of the horizontal, ground‐level magnetic field are determined. The radiation pattern is a combination of a linear dipole antenna and a right‐hand circular antenna. At ELF frequencies because of low D region absorption the dipole is dominant. The polarization and field strength in the near‐field can predict the orientation of the radiation pattern and the wave amplitude in the far‐field. The near‐field polarization indicates the direction and strength of the electrojet. As the electromagnetic wave propagates in the waveguide, it drives whistlers waves into the D region. Because of reflection from the D region the mode inside the waveguide is not purely transverse. This gives a counterclockwise rotation to the polarization. Directly above the heated region, waves are also launched along the Earth's magnetic field. The near‐field polarization shows good agreement with observations from the high‐power auroral stimulation array (HIPAS) and Tromsø.

Numerical comparison of two schemes for the generation of ELF and VLF waves in the HF heater‐modulated polar electrojet

Generation of ELF and VLF waves in the HF heater modulated polar electrojet is numerically studied. Illuminated by an amplitude modulated HF heater, the electron temperature of the electrojet is modulated accordingly. This, in turn, causes the modulation of the conductivity and thus the current of the electrojet. Emissions are then produced at the modulation frequency and its harmonics. The present work extends the previous one on a thermal instability to its nonlinear saturation regime. Two heater modulation schemes are considered. One modulates the heater by a rectangular periodic pulse. The other one uses two overlapping heater waves (beat wave scheme) having a frequency difference equal to the desired modulation frequency. It is essentially equivalent to a sinusoidal amplitude modulation. The nonlinear evolutions of the generated ELF and VLF waves are determined numerically. Their spectra are also evaluated. The results show that the signal quality of the second (beat wave) scheme is better. The field intensity of the emission at the fundamental modulation frequency is found to increase with the modulation frequency, consistent with the Tromso observations.

The effect of local irregularities on large-scale ionospheric currents

Measurements performed by the low-altitude satellites MAGSAT and TRIAD have discovered small-scale (L ⊥<80 km) magnetic fluctuations at high latitudes. A relation between the fluctuation intensity, ionospheric conductivity and Birkeland currents was also revealed. We assume that the magnetic perturbations are caused by small-scale ionospheric irregularities. We discuss here the influence of random ionospheric irregularities on the effective conductivity. Through analytical consideration, it is concluded that the Pedersen effective conductivity may noticeably deviate from the average conductivity. The effect depends primarily on conditions of Hall current closing. Wherever the Hall currents flow is impossible for any reason, the influence of weak irregularities is negligible. Zones of the equatorial and polar electrojets with electric field directed along electrojets are examples. Analytical consideration conducted for the regions exterior to these zones has shown that random weak ionospheric irregularities double the total Pedersen effective integral conductivity increases. It is usually suggested in the description of the ionospheric conductivity that the ionosphere is either homogeneous or exhibits local irregularity. The background sporadic irregularities in the ionosphere usually small scale is also well known. The research concentrated on two main items: the contribution of a random wind and conductivity field to the background and magnetospheric quasi-stationary electric and magnetic field, and the influence of small stochastic ionospheric irregularities on the effective ionospheric conductivity.

ELF/VLF radio signals caused by ionospheric demodulation of MF/HF radio transmitter signals

Abstract A quantitative theory of the generation of ELF/VLF signals, which are caused by the ionospheric demodulation of modulated signals from broadcasting MF/HF transmitters, is developed. Two cases are considered when the ionospheric current, modulated by broadcasting signals, is a one-dimensional linear current (the polar electrojet), and when it is an extended two-dimensional current sheet. It is shown that the main demodulation effect is caused by the formation of an ionospheric travelling wave antenna. In the case of the polar electrojet, this antenna radiates ELF/VLF waves in two directions, one of which coincides with the line connecting the broadcasting transmitter with the ELF/VLF receiver, and the other one is in a direction which is the mirror image of this line relative to the current direction. In this case the polarisation of the ELF/VLF magnetic field component varies widely. In the case of the extended ionospheric current, the demodulation takes place along the line connecting the MF/HF transmitter with the ELF/VLF receiver, and the horizontal ELF/VLF magnetic field component is normal to this line. Amplitude and phase distortions in the ELF/VLF signal are absent below a frequency ⪅ 1 kHz and appear above ∼ 1 kHz. Quantitative estimates of the amplitude of the ELF/VLF signal give a value ∼0.15pT which is close to the experimentally observed value.

A dynamic model for injection boundary of relativistic electrons in the earth's magnetosphere

The position of the outer belt maximum for relativistic electrons injected into the magnetosphere during magnetic storms is shown to depend on the largest amplitude for D st-variations for each storm. The dependence is shown to be of the form: |D st| max= 2.75 × 10 4 L 4 max· The relationship of the relativistic electron injection boundary to such structural magnetospheric elements as the solar proton penetration boundary and the polar electrojet position, etc. are analysed.

The latitude position dependence of the relativistic electron maximum as a function of Dst

The empirical dependence which defines the position of the outer belt maximum for relativistic electrons injected into the magnetosphere during magnetic storm (Lmax) on the largest Dst-variation amplitude for each storm |D st| max is studied. The dependence is shown to be of the form: |D st| max = 2.75 · 10 4 L 4 The injected particles intensity peak is formed in the transition region between the dipole field lines and the field lines extended pronouncedly to the magnetospheric tail. This region defines also position of various structural magnetospheric formations, namely, the polar electrojets, discrete auroral forms, the ∼1 MeV solar proton penetration boundary and the ring current maximum.

Review of ionospheric modification experiments at Tromsø

The most advanced results in previous heating experiments at Tromsø have been obtained in thefollowing research areas: 1. (1) Generation of secondary electromagnetic waves at very, extremely and ultra low frequencies by amplitude modulated heating of the polar electrojet current. 2. (2) Stimulated Electromagnetic Emissions (SEE) and HF wave anomalous absorption; particularly for heater frequencies near harmonics of the electron gyrofrequency. 3. (3) Excitation of Langmuir turbulence. Depending on altitude with regard to the pump reflection level, Langmuir turbulence may be represented either by propagating high- and low-frequency electrostatic waves, or by localized Langmuir states (cavitons) and density depressions (cavities). Typical experimental results pertaining to these items will be presented and discussed. Other experimental results will be briefly outlined.

Midlatitude detection of ELF whistlers

Narrow‐band, whistlerlike magnetic events distinguished by nearly monochromatic signals decreasing in frequency with time have been observed for the first time at midlatitudes in the ELF band. Measurements performed during September 3 to October 5, 1985 at Table Mountain, California (34.4°N, 117.7°W), show that the frequency and dispersion characteristics of these events are similar to events detected at auroral latitudes (Heacock, 1974), including a narrow‐band magnetic signal monotonically decreasing in frequency from 120 to 60 Hz over a 40 s interval with a mean center frequency of approximately 90 Hz. No echoes were observed. Maximum amplitudes of the magnetic signals ranged from just above the approximately 1 pT Hz−1/2 floor of the ambient background to roughly 20 pT Hz−1/2. The polarization was predominantly linear in the geographic east‐west direction. The midlatitude ELF whistlers reported here have a significantly lower average daily rate of occurrence than those reported for auroral latitudes. However, as with the high‐latitude events, they displayed an occurrence rate that is maximum during local daytime. Following Heacock (1974), it is suggested that a possible source for these events is whistler mode lion roars occurring in field‐aligned ducts of enhanced cold plasma densities in the distant magnetosheath. For favorable configurations of these ducts extending from the magnetosheath into the polar cusp, the waves may propagate to the Earth through the cusp acting as a waveguide. Although lightning is usually considered to be the dominant source of ELF noise in the Earth ionosphere cavity, magnetosheath ELF noise coupled into the cavity at high latitudes may represent an additional source. The fractional intensities of the natural ELF noise power within the cavity that are generated by this mechanism are presently unknown. However, a determination of these fractional intensities would be useful in the interpretation of the Schumann resonances as a measure of the totality of global lightning, particularly in the case of measurements made at high latitudes in the vicinity of the cusp‐ionosphere coupling region. Similarly, high‐latitude detection of very weak, artificially generated ELF signals, such as by HF heating of the polar electrojet, may be affected by the proximity of such a local ionospheric ELF noise source.

Collaborative experiments by Akebono satellite, Tromsø ionospheric heater, and European incoherent scatter radar

Joint experiments using the Akebono satellite and the ionospheric heating facility and European incoherent scatter radar near Tromsø were carried out in November 1990 and February 1991. In these experiments, Tromsø HF transmissions were amplitude modulated at frequencies of 2.5 and 4.0 kHz. Signals radiated from the polar electrojet (PEJ) antenna in the heated ionosphere at these VLF frequencies were detected for five passes out of seven passes of the semipolar orbiting Akebono satellite. A ground‐based observation of the VLF radiation from the PEJ was made in the November campaign at Lycksele in Sweden, 554 km south of the heating facility. Measurements of electron density profiles and dc electric fields in the ionosphere were carried out by the EISCAT incoherent scatter radar [Folkestad et al., 1983] located in the vicinity of the ionospheric heater. The results of the experiments are compared with those obtained by ray tracing and full‐wave analyses.

Full wave calculation of ELF/VLF propagation from a dipole source located in the lower ionosphere

A full wave technique has been developed to calculate the field intensities both in the upper ionosphere and on the ground when a dipole source immersed in the lower ionosphere radiates ELF/VLF waves. The radiated wave is divided into a large number of elementary plane waves, for each of which the propagation in the horizontally stratified model consisting of the ionosphere, the free space and the ground is calculated by the full wave technique. Then the plane waves are summed up to give a horizontal distribution of the radiated wave intensities at any altitudes. This technique is applied to investigate the propagation characteristics of radiation from a polar electrojet (PEJ) antenna modulated at an ELF/VLF frequency, which was created by the Tromsø heating facility. Comparison between the results of experiments and the calculation is given in the companion paper [Kimura et al., this issue]. We discuss several propagation characteristics, such as the frequency dependence of the radiated wave propagation up to the upper ionosphere and down to the ground. Under the conditions that the frequency is 2.5 kHz and the ionospheric dc electric field is 5 mV/m, the calculated results are roughly consistent with the experimental results and the radiation efficiency of the PEJ antenna is found to be extremely small, 2.5×10−6 upward and 3×10−8 downward.